The SEARCH ROM command helps to identify the number of devices and the 64-bit unique ID of each of the devices during the initial system bring-up when the host is not aware of the number of devices on the SDQ bus. The SEARCH ROM command sequence begins with the host generating the RESET pulse of at least 480μs. All the target devices on the SDQ bus responds with a PRESENCE pulse. Next, the host continues by issuing the SEARCH ROM command, F0h. Following the SEARCH ROM command, the host issues three time slots for each bit of the 64-bit serial ID as shown in Figure 6-11. In the first time slot, the target devices begins to transmit the bit of the 64 serial ID starting with the least significant bit. In the second time slot, the target devices transmit the complement of the bit. In the third time slot, the host writes the bit to be selected. The process continues until the end of the 64-bit serial ID. All the target devices that do not match the bit written by the host in the third time slot exits from the search process. If the host read zeros in the first two time slots, then a collision condition has occurred. The host knows there is more than one device on the SDQ bus. By choosing the bit value to write, the host is branching out to eliminate one or more contending target devices. At the end of the first run sequence, the host knows the serial ID of a single device. The sequence is then repeated to identify the serial IDs of the remaining devices on the SDQ bus.

Figure 6-11 SEARCH ROM Sequence